1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus or similar electrophotographic image forming apparatus and more particularly to a charge roller for an image forming apparatus and a method of producing the same.
2. Description of the Background Art
An image forming apparatus having at least one of a copier function, a printer function and a facsimile function includes a charger for uniformly charging the surface of an image carrier before the formation of a latent image. Japanese Patent Laid-Open Publication Nos. 3-240076 and 4-360167, for example, each disclose a charge roller made up of a shaft, an elastic member affixed to the circumference of the shaft, and a plurality of spacers mounted on the axially opposite end portions of the elastic member.
It has been customary to produce the charge roller by grinding the surface of the elastic member affixed to the shaft. The ground charge roller is pressed against the circumference of an image carrier with its portion between the spacers being spaced from the image carrier by a preselected gap. In this condition, a voltage is applied to the charge roller to thereby uniformly charge the surface of the image carrier. Subsequently, the charged surface of the image carrier is exposed imagewise to electrostatically form a latent image. A developing device develops the latent image for thereby producing a corresponding toner image.
The charge roller having the. above configuration reduces ozone and prevents substances contained in the elastic member from depositing on the image carrier and bringing about irregular charging. The charge roller therefore enhances the quality of the toner image.
If the gap between the charge roller and the image carrier is excessively great, the potential deposited on the image carrier is apt to noticeably deviate from a desired value and deteriorate the quality of the toner image. More specifically, each end portion of the elastic member has an outside diameter greater than the outside diameter of the intermediate portion, forming a rising portion. It has been customary to affix the spacers to the end portions of the elastic member without regard to the above fact. The rising portions, however, are likely to cause the gap between the elastic member and the image carrier to greatly deviate from a desired value even though the spacers may have a constant thickness, because the maximum diameter is not constant.
Moreover, when a voltage is applied to the charge roller, discharge is apt to occur between the edge of the rising portion where the outside diameter is maximum and the surface of the image carrier. This is because the edge protrudes toward the surface of the image carrier. The discharge brings about defective charging on the portion of the image carrier between the spacers and further aggravates the quality of the toner image.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication No. 2000-213529 and Japanese Patent No. 2,949,785.
It is an object of the present invention to provide a charger roller capable of preventing a gap between a charge roller and a body to be charged thereby from deviating from a desired value due to the rising portions of an elastic member, and a method of producing the same.
It is another object of the present invention to provide a charge roller capable of obviating discharge between a charge roller and a body to be charged thereby ascribable to the rising portions of an elastic member, and a method of producing the same.
It is a further object of the present invention to provide an image carrier unit and an image forming apparatus each including the above charge roller.
In accordance with the present invention, a charge roller includes a shaft, an elastic member affixed to the circumference of the shaft, and a plurality of spacers affixed to the elastic member. Assume that rising portions formed, when the circumference of the elastic member is ground during production, at axially opposite end portions of the elastic member each have a width of W mm in the axial direction. Then, the spacers each are positioned at a distance of at least W/9 mm from the end of the elastic member toward the center in the axial direction.
Also, in accordance with the present invention, a method of producing a charge roller including a shaft, an elastic member affixed to the circumference of the shaft, and a plurality of spacers mounted on the elastic member begins with a step of affixing the elastic member to the shaft. At least one of the elastic member and a grinder pressed against the elastic member is caused to rotate. The elastic member and grinder are caused to move relative to each other in the axial direction of the elastic member to thereby grind the circumference of the elastic member. A higher pressure is caused to act between the grinder and the elastic member when the grinder grinds opposite end portions of the elastic member in the axial direction than when it grinds an intermediate portion between the end portions.